1. Field of the Invention
A blowing agent is a chemical compound which decomposes on heating to a specific temperature to yield a vapor or gas or mixture of vapor and gas. In use, the blowing agent is incorporated in the thermoplastic material at a temperature below the decomposition temperature of the blowing agent and the mixture subsequently heated to a temperature above the decomposition temperature of the blowing agent whereupon the blowing agent decomposes to liberate a gas or vapor which forms small voids within the thermoplastic material. It is important that the blowing agent be finely and homogeneously dispersed within the thermoplastic material.
An important characteristic of a blowing agent is the temperature at which it decomposes to liberate gas or vapor since this temperature limits the temperature at which other operations may be performed on the thermoplastic material after incorporation of the blowing agent, but before actual formation of the cellular product. It is, therefore, often convenient to employ a blowing agent having a relatively high temperature of decomposition, this being especially the case when forming cellular products of thermoplastic material having relatively high fusion temperatures.
It is well known that the decomposition of a blowing agent should not occur until the processing temperature has been reached. The desired gas evolution should occur, however, prior to completion of the processing in order to produce a blown product of consistent density and also a minimum density for the blowing agent used. Also, the blowing agent should not react with or reduce the performance of any additives that are utilized in the processing of the materials being expanded.
Many of the blowing agents utilized in preparing cellular rubber or thermoplastic materials were unsatisfactory for use with materials such as polypropylene, polysulfonate, polysulfone, polyethylene oxide, polycarbonate and acrylonitrile-butadiene-styrene (ABS) due to the high temperatures required in processing these materials. Due to the fact that the processing temperatures on these materials were quite high, i.e., in the area of 100.degree. to 250+.degree. C. the blowing agents used prior to this invention had a tendency to become unstable prior to reaching the processing temperature and did not allow even distribution of the blowing agent nor uniformity of cell formation within the thermoplastics or thermoplastic blends.
2. Description of the Prior Art
Several patents disclose the use of compounds containing the tetrazole moiety as blowing agents. U.S. Pat. No. 3,366,581 discloses 5-hydroxytetrazole as a blowing agent for thermoplastic resins specifically claiming its use for polypropylene, high density polyethylene, polyamide, acrylonitrile-butadiene-styrene, and high temperature siloxane polymers. U.S. Pat. No. 3,442,829 discloses the use of 5-alkyl, cycloalkyl, arylalkyl, phenyl, and substituted phenyl tetrazoles in expanding a variety of high temperature processing resins. German Offenlegungsschrift No. 2,506,775 discloses the use of tetrazolylphthalimides in expanding polycarbonates and acrylonitrile-butadiene-styrene.